1. Field of the Invention
The present invention relates to a register mark detecting apparatus that can detect a transparent register mark printed in invisible ink on conveyed transparent web.
2. Description of the Related Art
When a multicolor pattern is printed on transparent web with a rotary printing machine, usually color patterns are overprinted one by one to form one pattern. At this point, when the color pattern is superimposed, sometimes pattern misalignment is generated or a white space is generated between the colors. Therefore, it is necessary that alignment be correctly performed such that register deviation is not generated. An automatic control apparatus called an automatic registering apparatus is used to perform the correct alignment. Usually, in the automatic registering apparatus, a register mark printed in each color in the transparent web is detected to compute a register deviation amount, and a compensator roller of a printing machine is controlled such that the register deviation is corrected.
As to a method of detecting the register mark printed on the transparent web, there is well known a method of optically detecting the register mark using a register mark detecting apparatus. The register mark detecting apparatus includes a light source such as a halogen lamp and LED and a light receiving element.
The register mark is classified into a register mark printed in visible ink and a register mark printed in invisible ink. In the case of the register mark printed in visible ink, the transparent web is irradiated with visible light to measure a change in quantity of light transmitted through the transparent web, which allows the register mark to be detected. On the other hand, in the case of the register mark printed in invisible ink, the register mark is hardly stably detected because a small difference in light quantity between the light transmitted through the register mark and the light transmitted through a portion in which the register mark is not printed.
For example, in a method disclosed in Patent Document 1, the register mark printed in invisible ink to which a small amount of fluorescent bleaching agent is added is irradiated with ultraviolet light to generate luminescence, thereby detecting the register mark. In a method disclosed in Patent Document 2, the register mark printed in invisible ink to which an ultraviolet absorber is added is irradiated with the ultraviolet light, and the register mark is detected from a change in quantity of light reflected or transmitted by ultraviolet absorption.
[Patent Document 1] Japanese Patent Application (Laid Open) No. S57-93154
[Patent Document 2] Japanese Patent Application (Laid Open) No. H1-283147
However, in case where additives such as a fluorescent bleaching agent and an ultraviolet absorber are added to the invisible ink like Patent Documents 1 and 2, possibly an appearance of a printed paper is adversely affected. Because the use of the light source emitting the ultraviolet light is required, cost of the apparatus is increased compared with the use of the visible light source.